1. Field of the Invention
The invention relates to a process and an apparatus for the lateral relaxation of stresses at the end of oedometric compacting of a block constituted by an aggregate and a binder shaped by vibratory tamping or ramming.
It applies more particularly, but not exclusively, to the production of carbon-containing blocks, constituted by coke and/or anthracite (aggregate) and pitch (binder) used in electrometallurgy, e.g. anodes and cathode elements to be used in vessels for the production of aluminium by electrolysis.
2. Description of Related Art
Oedometric compacting of a block or shaped member by vibratory tamping or ramming consists of placing the material to be shaped in a mould equipped with a cover and imparting thereto one or more stress cycles. These stresses are applied by the repeated dropping of a weight onto the material (ram or heavy cover).
In order to facilitate understanding of the invention, it is pointed out that vibratory tamping is obtained by the use of a heavy cover and the application to the mould of vibrations at a frequency between a few cycles and a few dozen cycles, which has the effect of raising the cover and which then drops under the effect of its own weight onto the block to be compacted.
Ramming consists of applying repeated shocks or impacts to the block by means of a ram, which is raised and allowed to fall in a periodic manner.
FIGS. 1, 2 and 3 relate to the prior art.
FIGS. 4, 5, 6, 7 and 8 relate to the invention.
All the drawings are diagrammatic sections and relate to the particular case where the main stress is exerted vertically from top to bottom, but still apply no matter what the position of the mould in space.
FIG. 1 shows that the main stress Fa is exerted vertically on the material (1) in mould (2). The mould sidewalls (3) are fixed and exert on the material a reaction Fl proportional to the main stress: Fl=k.times.Fa.
Hereinafter, the term "lateral stress" will be used to define any stress exerted in a direction perpendicular or substantially perpendicular to the direction of the vertical stress.
At the end of compacting (which can e.g. last a few minutes), cover (4) is removed and the material (1) which had stored the lateral stresses will now release them. The sidewalls of mould (3) are fixed, so that the stresses can only be released in the direction of the compacting axis and can lead to the formation of cracks (6) perpendicular to the compacting axis. This is e.g. what occurs during the moulding of anodes for the production of aluminium by electrolysis, when the carbon-containing paste in the mould exceeds a certain critical temperature.